This invention relates to a passive SSR (Secondary Surveillance Radar) system for monitoring the position of an aircraft by intercepting replies which an issued from the aircraft in response to an interrogation from SSR station.
As is well known, an aircraft in airborne is obliged to send out a desired information via a transponder when it receives an SSR interrogation from each of SSR stations located disposed in many places. Conventionally there is a passive SSR system for detecting the position of a desired aircraft upon receipt of both interrogation from SSR station and replies from the desired aircraft.
According to the detection principles of the above passive SSR system, if a distance between an SSR station and an aircraft is R1 and a distance from the aircraft to a receiving position X is R2 as shown in FIG. 13, the propagation time of interrogation and reply corresponds to the distances (R1+R2). In other words, an aircraft is located at one point on an ellipse whose two focuses correspond to the SSR station and the receiving position X and whose longer diameter corresponds to the distances (R1+R2), and the position of the aircraft on a two-dimensional plane is detected based on an angle xcex8 between a straight line connecting the SSR station and the receiving position X and a straight line connecting the SSR station and the aircraft.
The above passive SSR system is under restrictions on use environment since it has difficulty in detecting the position of an aircraft when it is difficult for the system to receive an interrogation from an SSR station with high precision because of high mountains or buildings between the receiving point and the SSR station.
As described above, the problem of the conventional passive SSR system is that the position of an aircraft is difficult to detect unless the aircraft can intercept an interrogation from the SSR station.
The present invention has been developed in consideration of the above situation and its object is to provide a passive SSR system which is simple in configuration and capable of detecting the position of an aircraft with high precision without receiving any interrogation from an SSR station.
A passive SSR system according to one aspect of the present invention comprises reply receiving means for receiving a reply issued from an aircraft in reply to an interrogation sent out from an SSR station, calibration means for detecting transmission timing of the interrogation sent out to the aircraft from the SSR station, antenna directly-facing timing of the SSR station, and interrogation patterns, based on the reply from the aircraft for calibration received by the reply receiving means, aircraft position detecting means for detecting a position of the aircraft based on the reply and the transmission timing and the antenna directly-facing timing of the SSR station detected by the calibration means, and monitor means for monitoring the position of the aircraft detected by the aircraft position detecting means.
In the passive SSR system described above, reply is received from an aircraft for calibration which is issued in reply to an interrogation from an SSR station, transmission timing of the interrogation from the SSR station and antenna directly-facing timing are detected based on the reply, and a position of the aircraft on a two-dimensional plane is detected based on the transmission timing, the antenna directly-facing timing and the reply from the aircraft. Consequently, the position of the aircraft can be detected with high precision even in a place where an interrogation is difficult to receive from the SSR station.
A passive SSR system according to another aspect of the present invention comprises reply receiving means for receiving a reply issued from an aircraft in reply to an interrogation sent out from an SSR station, calibration means for specifying the aircraft, which sends out the reply to the reply receiving means, based on aircraft position information from ADS (Automatic Dependent Surveillance), and detecting transmission timing of the interrogation sent out to the aircraft from the SSR station, directly-facing timing of an antenna of the SSR station, and interrogation patterns, based on the reply, a site of the SSR station and the aircraft position information, which are stored in advance, aircraft position detecting means for detecting a position of the aircraft based on the reply and the transmission timing and the directly-facing timing of the antenna of the SSR station, which are detected by the calibration means; and monitor means for monitoring the position of the aircraft detected by the aircraft position detecting means.
In the passive SSR system described above, while aircraft positional information is input from the ADS, an aircraft, which is sending out a reply in reply to an interrogation from an SSR station, is specified, and both transmission timing of the interrogation from the SSR station and directly-facing timing of an antenna of the SSR station are detected from a position of the aircraft and reception timing of the reply. Furthermore, time lag is obtained from time data of the reception timing of the reply and the aircraft position information, and the transmission timing of the interrogation from the SSR station and the directly-facing timing of the antenna are corrected based on data of the time lag, thereby detecting a position of the aircraft based on the transmission timing and directly-facing timing and the reply from the aircraft. Consequently, the position of the aircraft can be detected with high precision even in a place where an interrogation is difficult to receive from the SSR station.
A passive SSR system according to still another aspect of the present invention comprises reply receiving means for intercepting a reply issued from an aircraft in reply to an interrogation generated from an SSR station, calibration means for storing generation time information of the interrogation of the SSR station and directly-facing time of an antenna of the SSR station, which are preset, and detecting transmission timing of the interrogation sent out to the aircraft from the SSR station and antenna directly-facing timing of the SSR station based on time of a common clock, aircraft position detecting means for detecting a position of the aircraft based on the reply and the transmission timing and the antenna directly-facing timing of the SSR station, which are calculated by the calibration means, and the reply, and monitor means for monitoring the position of the aircraft detected by the aircraft position detecting means.
In the passive SSR system described above, both transmission timing of an interrogation of an SSR station and antenna directly-facing timing of the SSR station are detected based on generation time information of the interrogation of the SSR station and antenna directly-facing time information of the antenna, which are stored in advance, and time of a common clock, thereby detecting a position of the aircraft in reply to the transmission timing, antenna directly-facing timing, and the reply from the aircraft. Therefore, the position of the aircraft can be detected with high precision even in a place where an interrogation is difficult to receive from the SSR station.
The calibration means according to the present invention is so constituted that an aircraft, which is sending out a reply to the reply receiving means, is specified based on aircraft position information from ADS and transmission timing of the interrogation sent out to the aircraft from the SSR station is detected based on the reply, the aircraft position information, and the positional information of the SSR station.
According to the above constitution, time lag between the reply and aircraft position information is detected, the transmission timing is corrected based on the time lag, and at least transmission timing is detected by the corrected timing, with the result that higher precision transmission timing can be detected.
A passive SSR system according to yet another aspect of the present invention comprises reply receiving means for receiving a reply issued from an aircraft in reply to an interrogation sent out from an SSR station, calibration means for, upon externally receiving transmission time information of an interrogation of the SSR station and antenna directly-facing time information of the SSR station which are based on time of a common clock, correcting the transmission time information and the antenna directly-facing time information based on the time of the common clock, and detecting transmission timing of the interrogation sent out to the aircraft from the SSR station and antenna directly-facing timing of the SSR station, aircraft position detecting means for detecting a position of the aircraft based on the reply and the transmission timing and the antenna directly-facing timing of the SSR station detected by the calibration means, and monitor means for monitoring the position of the aircraft detected by the aircraft position detecting means.
In the passive SSR system described above, if generation time information of an interrogation of the SSR station and antenna directly-facing time information of the SSR station, which are based on the time of a common clock, is externally supplied, they are corrected by the time of the common clock to detect transmission timing of the interrogation sent out to the aircraft from the SSR station and antenna directly-facing timing of the SSR station and thus detect a position of the aircraft based on the transmission timing, antenna directly-facing timing and the reply from the aircraft. Consequently, the position of the aircraft can be detected with high precision even in a place where an interrogation is difficult to receive from the SSR station.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.